NTSB report says wheel slide led to collision between CTA train, snow removal machine

WASHINGTON — Wheel slide, reflecting a trio of factors, led to the collision between a Chicago Transit Authority Yellow Line train and a snow-removal machine involved in training, the National Transportation Safety Board said in its final investigation report on the Nov. 16, 2023, accident, released today (Feb. 5, 2025).

The wheel slide reflected an “aggressive speed reduction command;” a Chicago Transit Authority decision to disable the automatic track brake application feature of the train’s wheel slide protection system; and organic material on the rails that caused slippery conditions, according to the report.

The incident occurred about 10:30 a.m. local time near the Howard station. The Yellow Line train (known informally as the Skokie Swift) was carrying an operator and six passengers; six CTA employees were on the snow removal machine, which was stationary at the time of the collision. Three people, including the train’s operator, suffered critical injuries; 16 others were transported to hospitals where they were treated and released. Reports at the time said at least 38 people were injured, but some declined treatment at the scene [see “Chicago L train hits maintenance equipment …,” Trains News Wire, Nov. 16, 2023].

The collision led to a 50-day shutdown of the 4.7-mile Yellow Line while the CTA imposed new operating rules and reduced top speeds from 55 mph to 35 mph [see “CTA to resume Yellow Line operations …,” News Wire, Jan. 4, 2024]. The CTA said the collision resulted in an estimated $8.7 million in equipment damage.

The snow-removal machine was stopped about 370 feet north of a red signal at the time of the accident when the Yellow Line train, No. 593, received an in-cab stop command from the automatic train control system because of the presence of the snow equipment. That command came with the train traveling about 54 mph and about 2,150 feet from the snow-removal machine.

The train’s operator initiated a full-service brake appliation about one second later, and initiated emergency braking about 18 seconds after that, upon sighting the snow-removal machine. The train had slowed to about 23 mph when it struck the snow removal machine some 28 seconds after the initial stop command.

NTSB investigators found train No. 593’s dynamic and friction brakes were applied during the full-service brake application, but that the track brakes did not. The CTA had found during testing of the 5000-series railcars such as those involved in the collision, the cars’ wheel slide protection system tended to continuously deploy the track brakes, which could produce a high brake rate uncomfortable to passengers. Automatic application of the track brake had therefore been disabled, with use of that brake left to the discretion of the operator.

Investigators found that braking began with stopping distance greater than the 1,780 feet needed under ideal conditions, and in stopping tests, were able to stop the train within that distance. However, those tests were not able to replicate the combination of conditions, particularly the slippery organic material on the tracks. “Direct comparison between ideal and relevant non-ideal conditions was not possible,” according to the report.

Toxicology results shows the operator had consumed ethanol prior to the accident and was likely experiencing some impairment, but given the response to the stop command within one second, the NTSB determined it was unlikely the effects of the ethanol contributed to the collision.

As a result of the accident, the CTA has re-enabled the automatic track brake feature on the 5000-series railcars; cleaned the organic material on the rails believed to have contributed to the wheel slide; modified its track inspector certification program to identify material build-up on the rails; and reduced speeds on the Yellow Line to eliminate the possibility of a 55-mph-to-0 speed reduction.